r = rand;
will give a positive number between 0 and 1. It will never be negative. You'll have to ask your teacher why he or she rejected that. Perhaps he/she wanted it multiplied by something, like r needs to go between 0 and 15:
r = 15 * rand(1);
Maybe you didn't write a function but just wrote a script. Maybe the function needed to have input and output arguments and you didn't give them. It's hard to say without seeing (1) your code that you showed your teacher, (2) the problem statement, and (3) your exact teacher comments. You'll have to ask your teacher for clarification.
